Can you ever have too much of a good thing? Audiences don't seem to think so. Advertise an event that lasts the best part of a day and you will find the box office quickly besieged. Having sat through a couple of epics in the last fortnight – Gatz at the Noel Coward and Druid Murphy at the Hampstead – I not only have a new understanding of the phrase "putting in an eight-hour day" but have come to several conclusions.

One: epics tend to disarm criticism. It's partly because they create a sense of collusion between cast and spectator that you don't find in the average play. But it's also because audiences tend to bond in all sorts of ways. By the time you have gossiped, exchanged email addresses and even, in some instances, nodded off alongside your neighbours, you tend to feel less like a critic and more like part of a larger community.

Two: there is always a moment, usually after the meal-break, when energy levels dip both on stage and in the auditorium. It happened in the last segment of Gatz, an eight-hour reading of The Great Gatsby novel; and in Famine, the third of the three Tom Murphy plays in the nine-hour Druid Murphy show. Audiences, like actors, need to be fed and watered but, once the spell has been broken, it takes time to re-immerse yourself in an imaginary world.

Three: epics demand serious material. Over the years, I've watched day-long events involving the English nobility carving itself to pieces in The Wars of the Roses, as well as conflict between good and evil in The Mahabharata. Neither was without humour, but a trio of classic farces would be insufferable. They would probably succeed all too well in their aim of making audiences die laughing.

Four: epics satisfy our current hunger for event theatre. We love the inordinately long, the cryptically short or anything with star names. What we seem to have lost is our taste for anything good of average length.

Five: appetite increases with what it feeds on. I'm off to the Royal Opera House to see Berlioz's Les Troyens – clocking in at five-and-three-quarter hours. Will I, I wonder, find it a bit too short?